1. Field of the Invention
This invention relates generally to the field of aircraft flight control systems and more particularly to a Mid-Value Selection function for incorporation in multichannel fly-by-wire system.
2. Description of the Related Art
Prior to the Boeing 777 aircraft Fly-By-Wire (FBW) systems, the most complicated flight controls computers in the prior art were triple channel autopilot computers for legacy Boeing airplanes (Boeing 747, 757, 767). On these flight controls systems, mechanical/hydraulic actuators serve as the mid-value selector for the control of airplane primary control surfaces, with electrical input commands from triple autopilot computers. The mechanical/hydraulic mid-value selector is less reliable and maintainable, and is more costly compared to electronics flight controls, such as those employed on the Airbus 320 and Boeing 777 FBW airplanes.
As disclosed in U.S. Pat. No. 5,802,077 entitled Methods and Apparatus for Obtaining High Integrity and Availability in a Multi- Channel System, issued Sep. 1, 1998 to Ying C. Yeh and U.S. Pat. No. 5,515,282 entitled Methods and Apparatus for Implementing a Databus Voter to Select Flight Command Signals from One of Several Redundant Asynchronous Digitial Primary Flight Computers issued May 7, 1996 to Douglas O. Jackson, both patents having a common assignee with the present application and incorporated herein by reference as though fully set forth, the conclusion reached for systems for the Boeing 777 Fly-By-Wire Architecture was to design an asynchronous multi-channel (a minimum of three channels with a minimum of three computation lanes in each channel) system as the host to serve as the guardian of the common communication media. The three computation lanes in each channel employed dissimilar processors and compilers so that the computer architecture is fail-operational to generic errors.
It is desirable for a new architecture to provide a lower cost solution for FBW computer architecture based on prior art FBW experience. It is also desirable that the prior art Primary Flight Computer (PFC) mid-value selection function be replaced by a mid-value selection function integrated in each digital/analog conversion channel generally referred to herein as Actuation Control Electronics (ACE). On exemplary prior art FBW systems, the PFC can command 50% of primary surface and therefore the mid-value selection function as identified in U.S. Pat. No. 5,515,282 is designed with a functional integrity of 1.0E-10 per hour. It is possible for new FBW systems, with mid-value selection in the ACE and with each ACE controlling 25% of primary surface, that the mid-value select function can be designed with a lesser integrity requirement than that of the prior art PFC equivalent function.